As having the advantages of impact resistance, heat resistance, good electric properties and transparency, aromatic polycarbonate resins are used in various fields. In particular, because of their excellent transparency, they are favorably used for various applications of optical devices such as optical lenses and optical waveguides, and also for optical information-recording media, display panels, illuminator covers, and other substitutes for glass. In some applications thereof, however, the transparency of aromatic polycarbonate resins is often unsatisfactory.
For example, backlight units for liquid-crystal image displays and those for various guide lights generally have a built-in surface light source of a transparent tabular molding that emits light uniformly. The transparent tabular molding receives the light from a main light source, cathode ray tube (fluorescent lamp) combined with it, and emits light from its surface, and this is referred to as an optical waveguide. The material for such an optical waveguide must not attenuate light that passes through it, and is preferably lightweight and well workable.
From these viewpoints, it has heretofore been said that polymethyl methacrylate (PMMA) of thermoplastic resins is the most suitable for optical devices. The overall parallel light transmittance through PMMA is on a high level, falling between 91 and 93%, and the transparency of PMMA is extremely high. In view of its good transparency and workability, therefore, PMMA is an extremely excellent resin for optical devices. However, the heat resistance, the impact resistance and the flame retardancy of PMMA are not always satisfactory. Therefore, the problem with PMMA is that its service conditions are limited for optical waveguides, display panels and illuminator covers.
For example, optical waveguides for backlights for instrument panels, tail lamps and winkers for automobiles must satisfy the requirements of thermal deformation resistance at 120° C. or higher and falling weight impact strength of at least 10J, and for these, PMMA is often impracticable.
In their practical use, aromatic polycarbonate resins have no problem in point of the heat resistance and the impact resistance thereof, but their transparent is far inferior to that of PMMA. Therefore, for optical applications such as optical waveguides, etc., it is desired to increase the level of the transparency of aromatic polycarbonate resins.
Various compositions prepared by blending an aromatic polycarbonate resin and an acrylic resin of higher transparency have been proposed. In general, a resin composition that comprises polycarbonate and polymethyl methacrylate could not be a uniform transparent resin, and it has heretofore been developed as a specific opaque resin having a pearly gloss. After that, some transparent resins have become developed, taking the advantages of the two resins.
For example, <1> JP-B 56-28937 discloses a composition of a polycarbonate and a low-molecular acrylic copolymer that comprises from 75 to 90% by mass of methyl methacrylate and from 10 to 25% by mass of alkyl acrylate. However, in order that the composition could be transparent, the molecular weight of the acrylic copolymer in the composition must be at most 15000, and the acrylic copolymer serves as a plasticizer in the composition. The problem with the composition disclosed is that the physical properties of the polycarbonate resin in the composition are significantly worsened. Though not having any negative influence thereon, the acrylic copolymer in the composition does not improve the transmittance of the composition.
<2> JP-A 63-90551, 63-256647 and 64-1749 say that a copolymer of methyl methacrylate, mono-substituted (meth)acrylamide, maleinimide and (meth)acrylate having a carbon cyclic group is miscible with polycarbonate to give a transparent composition. However, the composition is transparent only when it forms films, but is not transparent when it forms moldings having a thickness of a few mm. Therefore, the composition is not applicable to optical materials and optical waveguides.
To solve the problem, <3> JP-A 4-359953 and 4-359954 disclose an improved composition of an aromatic polycarbonate and a methacrylate copolymer, in which the methacrylate copolymer has at least 50% by mass of phenyl methacrylate units. However, as is obvious from Examples and Comparative Examples given in these, the haze (%), one index of transparency, of 2-mm sheets of the composition that contains 3% by mass of methacrylate copolymer is from 5 to 8%, while, on the other hand, the haze (%) of the same sheets of polycarbonate alone is 4%. This means that the transparency of polycarbonate resin is rather lowered when methacrylate copolymer is added thereto.
As in these, essential improvement of transparency of polycarbonate resin is in fact difficult even if polycarbonate resin which is transparent by itself is combined with acrylic resin having higher transparency.
<4> JP-A 10-73725and 10-158364 disclose a polycarbonate resin composition of good light transmittance, which comprises 100 parts by mass of polycarbonate resin and from 0.001 to 1 part by mass of acrylic resin and in which the molecular weight of the acrylic resin preferably falls between 200 and 100,000. The composition disclosed differs from the other conventional compositions in point of the technical idea in that the amount of the acrylic resin to be added to polycarbonate resin is at most 1 part by mass, and, in addition, the photoconductivity of the composition is good. Concretely, even when 0.2 parts by mass of acrylic resin is added to 100 parts by mass of polycarbonate resin, the photoconductivity of the resulting composition is comparable to that of acrylic resin alone. In addition, the other advantages of the composition are that the composition has high impact resistance and good heat resistance intrinsic to polycarbonate resin.
However, in the field of optical devices, further improvement of the transparency of polycarbonate resin is much more desired. The composition <4> almost satisfies the transparency and the photoconductivity of acrylic resin of an ordinary grade and is comparable to such an ordinary-grade acrylic resin, but is still inferior to high-transparency acrylic resin for optical use.
Given the current situation as above, the present invention is to further improve the transparency of aromatic polycarbonate resins, and its object is to provide an aromatic polycarbonate resin composition that gives moldings comparable to those of acrylic resins for optical use in point of the transparency not losing the characteristics of good impact resistance and heat resistance intrinsic to aromatic polycarbonate resins, and to provide the moldings of the resin composition.